Examination of an eye portion is widely performed for the purpose of preemptive medical care for lifestyle-related diseases and other diseases that are major causes of blindness. A scanning laser ophthalmoscope (SLO) serving as an ophthalmic apparatus based on the principle of a confocal laser microscope performs raster scanning on a fundus with a laser beam serving as measurement light, and quickly obtains a high-resolution planar image based on the light intensity of the return beam. Such an apparatus for capturing a planar image will be referred to as a SLO apparatus hereinafter.
In recent years, it has become possible to obtain a planar image of a retina with an improved lateral resolution by increasing the beam size of measurement light in a SLO apparatus. As the beam size of measurement light increases, however, the resolution and SN ratio of a planar image decrease due to aberration of an eye to be examined in obtaining a planar image of a retina.
To solve the problem, an adaptive optics SLO apparatus has been developed that includes an adaptive optics for causing a wavefront sensor to measure the aberration of an eye to be examined in real time, and causes a wavefront correction device to correct the aberration of measurement light and to examine the return beam occurring in the eye. Such an adaptive optics SLO apparatus can obtain a high-lateral resolution planar image.
Furthermore, it is possible to obtain such a high-lateral resolution planar image as a moving image (to be referred to as a SLO moving image hereinafter), thereby enabling to noninvasively measure, for example, the hemodynamics of retinal capillaries. More specifically, an image of luminance variations in the frame direction at each x-y position of a SLO moving image is obtained, and retinal capillaries are extracted (FIG. 5B), thereby determining a blood flow velocity measurement position. After that, the measurement position (denoted by a reference symbol pt in FIG. 5A) is specified on the SLO moving image to generate a curved cross-sectional image (FIG. 5D) called a time-space image on the path. The movement locus of blood cells is detected from the time-space image, and a blood cell moving speed is measured based on the angle of the locus.
In the above blood flow measurement processing, an exceptional frame for which measurement processing is difficult due to differences in image features caused by an imaging apparatus or the influence of eye/eyelid movement may occur. Especially for a diseased eye, in many cases, a low luminance frame may occur due to blinking as shown in FIG. 5A, and a frame obtained by capturing an area other than an imaging target area may occur due to fixation disparity. Furthermore, a moving image may include a frame with a low SN ratio due to the characteristics of an apparatus such as an aberration correction failure. If measurement processing is performed without consideration of the influence of such exceptional frames, an image of blood vessels blurs in extracting the blood vessels as shown in FIG. 5C, thereby disabling to specify the positions of the specific blood vessels. Alternatively, in some cases, as denoted by reference symbols Eb and Ed in FIG. 5E, information of a low luminance area or different imaging position is contained in the time-space image due to blinking or fixation disparity, and the locus of blood cells to be measured breaks, thereby making it difficult to detect the movement locus of the blood cells.
To prevent an exceptional frame from occurring in a SLO moving image, there is provided a method of including, in an apparatus, a tracking mechanism for preventing an exceptional frame from occurring in an imaging operation. It is, however, necessary to additionally provide an arrangement for capturing a wide field of view SLO image. Even if such a tracking mechanism is provided, a complete tracking operation is actually difficult when involuntary eye movement during fixation is large.
A technique is required to automatically determine exceptional frames in a SLO moving image to perform image measurement by excluding the exceptional frames or to perform image measurement by changing an image processing method in frames close to the exceptional frames. Japanese Patent Laid-Open No. 2007-330582 (to be referred to as literature 1 hereinafter) describes, as a method of measuring a blood flow velocity in an eye portion moving image, a technique of specifying the start and end points of a blood vessel of the fundus, and obtaining a blood flow velocity based on the time difference in luminance between the start and end points.
In the technique described in literature 1, however, it is necessary to correctly irradiate a blood vessel portion with a beam. If the fundus moves, it is impossible to measure a blood flow velocity. Literature 1 does not consider a case in which a SLO moving image includes a frame obtained when the fundus moves.
Furthermore, a technique of extracting blood capillaries based on the standard deviation of the luminance values between moving image frames of an aberration correction SLO moving image is described in J. Tam et al. “Noninvasive Visualization and Analysis of Parafoveal Capillaries in Humans”, IOVS, Vol. 51, No. 3, pp. 1691-1698, 2010 (to be referred to as literature 2 hereinafter). In literature 2, since the standard deviation of the luminance values is obtained using all the frames, an image of blood vessels blurs when the fundus moves (the value of the standard deviation becomes large due to a factor other than movement of blood cells). That is, literature 2 does not consider a technique of correctly extracting blood capillaries when a frame obtained when the fundus moves is included, either.